Cotton and other similar plant growers have difficulty controlling weeds in or near the rows of the plants. Typically, the crop is planted in elongated rows having a depressed furrow therebetween. The traditional hand hoeing of weeds has become expensive and is often not effective. Some control of weeds in or near the rows may be achieved by using a cultivator to throw soil up and onto the row and thereby bury small weeds, however, large or hardy weeds are not eradicated by such "dirting".
Growers have also used "liquid hoes" or "chemical hoes" to destroy weeds growing in the row. A liquid hoe consists of a spray nozzle on each side of the row of plants, the nozzle spraying herbicide upon the top of the row at the weeds and soil beneath the crop foliage. Properly done, the spray streams pass beneath the plant foliage and slightly overlap each other at the center of the crop row. If the herbicide used will also damage the crop, care must be taken not to spray the crop foliage. A liquid hoe application should result with all of the soil within four to six inches of the row having been sprayed with herbicide with little or no damage to the crop.
Shields are used with the liquid hoes to protect the crop from herbicide damage by lifting and/or pushing crop foliage away from the spray stream as the apparatus moves forward. The shield also provides protection by keeping the spray mist from drifting onto the crop. Shields can increase the efficiency of a liquid hoe by lifting low crop foliage so that the spray stream can contact soil and weeds which would otherwise be missed.
However, lack of precision alignment of the herbicide spraying apparatus to the crop row has been the major problem with liquid hoes. Misalignment of the spraying mechanism results in crop damage from the herbicide contacting the crop, or from the apparatus itself tearing into the crop, damaging the crop and missing the weeds to which the spray is directed. Because the spray nozzles and shields are mounted in a fixed orientation on the tool bar held by the tractor or self-propelled sprayer, the operator must precisely guide the apparatus along the rows and regulate the height of the apparatus.
There have been devices guiding the spraying apparatus by sensing the bottom of the furrow and/or the slope of the bed upon which the crop row is planted. However, problems readily arise in that the furrows of a field often vary in depth, size, shape, and symmetry. Thus an apparatus which aligns itself by sensing the furrow can not do a precise job in a field with non-uniform furrows and beds. In addition, large crop plants make the spray application by present liquid hoes even more difficult because the operator can not see under the crop foliage.
One other problem which readily presents itself to apparatus sensing the furrow or the sides of the crop row bed is that on opposite sides of each group of crop row beds and furrows is what is known as the "guess-row". The reason for this is that it is not possible nor economical to plant all of the seeds in rows in a field with an exact distance separation. For example, a tractor will pull a four row crop planter across the field. The distance between each of the four seed openers on the planter are fixed at the desired row spacing by the machinery. Thus the row spacing between each row of crop plants is fixed for four rows. However, when the operator turns the planter around at the end of the field and commences the next pass of planting the seeds, the distance between the last row of planted seeds and the adjacent first row of the next pass is not fixed and in fact, will vary along the row. The operator will achieve some measure of ability in crop row spacing, however, it is common that the row spacing between two planter passes may vary along the rows as much as plus or minus 10% of the desired row spacing. Growers refer to this variable distance as the "guess-row".